You want to operate on the cell elements, not on scalar cells:
if stock(b) > stock(b+1)
% ==>
if stock{b} > stock{b+1} % Curly braces!
This works, if both strings have the same length. A conversion to a numerical is not needed. If the strings have different length, pad the shorter one with zeros:
char0 = char(0);
s1 = stock{b};
s2 = stock{b+1};
len1 = length(s1);
len2 = length(s2);
if len1 > len2
s2 = [s2, char0(1, len1 - len2)];
elseif len2 > len1
s1 = [s1, char0(1, len2 - len1)];
end
if s1 > s2 ...
